The need for energy security and energy surety is slowly driving worldwide end-energy users towards renewable and alternative sources of energy. The impact of this movement appears to have a correlation with progressively reduced cost of the energy sources. A further need is to reduce the cost of the interfacing power-electronics system (PES), which is relatively difficult since power electronics is already a relatively mature field.
As a nonlimiting example, high efficiency, low cost, and high power density are important attributes of a multi-phase converter (i.e., converter with one or more phases) fed with a dc source. Nonlimiting example applications include distributed generation (DG) systems with renewable and alternative energy sources (e.g., photovoltaic arrays, wind with front-end rectifier or fuel-cell stacks), energy storage systems, microgrid, vehicle-to-grid applications, electric/hybrid-electric/fuel-cell vehicles, compact power conversion modules (PCMs) for naval, space, and aerospace applications, and battery-based uninterruptible power supplies (UPSs). In such systems, galvanic isolation is often required for safety concerns and voltage and current scalabilities.
In that regard, an improvement in the art has resulted in a shift away from line-frequency-transformer-based bulky inverters towards high-frequency-transformer-based inverters, which saves weight, volume, footprint space, and labor cost. This first phase has resulted in about 20-kHz isolated inverters with efficiencies around 90%.
However, it is being realized that to reduce the cost even further, the inverter design should have a universal element to it; i.e., a technology that is applicable not only to photovoltaic or fuel cell (PV/FC/wind) type sources, but also suitable for other traditional inverter applications with comparable specifications. The power density of the inverters should operate at higher switching frequency without sacrificing efficiency and without adding significantly to the cost. Approaches to reduce filter size or eliminate them without compromising performance factors would provide additional benefits.
Among possible topologies, a high-frequency-link (HFL) pulse-width-modulated (PWM) converter can eliminate the intermediate LC filter that is needed for a conventional high-frequency (HF) fixed-dc-link converter approach. Further, as compared to a resonant-link inverter, it yields lower switch stress, better total harmonic distortion (THD), and simpler all-device structure (e.g., no passive components in power stages). Thus, the PWM HFL converter approach is better suited from the viewpoints of cost, efficiency, and portability.
One HFL converter topology in the art is a cycloconverter-type HFL (CHFL) converter, which reduces conversion complexity by directly placing a cycloconverter to the secondary side of an HF transformer. Another topology is a rectifier-type HFL (RHFL) converter. The RHFL possesses a structure similar to that of a conventional fixed-dc-link converter except for the absence of the de-link filter. It can be thought of as a distributed version of the CHFL topology.
Cycloconverter-type inverters eliminate intermediate dc-link filters, and thus reduce parts count. However, operation of such a scheme involves rectifier diodes, which experience reverse recovery losses if the inverter operates at high switching frequencies. Furthermore, the output voltage and output current polarities need to be sensed, as two different switching schemes are required for unity and non-unity power-factor loads. This is because, for the latter case, there are time durations during which the output voltage and output current have opposite signs. This leads to complex operational logic. Further, the differences in the switching actions makes it apparent that additional switching for the non-unity-power-factor case leads to additional switching losses for the ac/ac converter, which can limit efficient operation at much higher frequencies of inverter operation.